There are two types of acetaldehyde (AA) of concern. The first is residual or free AA contained in the pellets or particles sent to preform molders. The second type of AA is preform AA or the AA generated when the PET pellets are melt processed to make bottle preforms. AA precursors in the pellets can be converted to AA upon melting and give unacceptable levels of AA in the preforms. Melt processing also forms more AA precursors, which can liberate AA. Acetaldehyde has a noticeable taste and can be detected by human taste buds at low levels. Unacceptably high AA levels in preforms are those that adversely impact the taste of the beverage contained in the said bottles or those that exceed brand owners' specifications for the desired application.
Relatively tasteless beverages such as water are particularly negatively impacted by the strong taste of AA. Many water bottle applications require lower levels of perform AA than carbonated soft drink (“CSD”) bottle applications. Converters who take polyester particles and make bottle preforms would like to have one resin that could be used to make preforms for both water and CSD applications. This would simplify the materials handling process at the converter by allowing for one feed silo or one type of feed silo for water and CSD applications, one product storage area or one type of product storage area for water and CSD applications, etc. Most resins sold into water bottle markets have a lower It.V. than those resins sold into CSD markets. A dual use resin would have to a high enough It.V. for CSD applications and a low enough level of preform AA for water bottle applications. The level of perform AA is impacted by the AA generation rate of polyester particles upon melting.
In order to use one resin, some converters are adding AA scavengers to CSD resins to get acceptable perform AA for the water bottle market. AA scavengers add significant cost to the container and often negatively impact the color of the container by making it either more yellow or darker as compared to an analogous container without AA scavenger added. Certain carriers for AA scavengers hurt preform appearance, and subsequently bottle appearance, due to more or larger black specks relative to an analogous preform or bottle without AA scavenger and its carrier added.
There is cost savings associated with manufacturing polyesters entirely in the melt phase. There are also subsequent melt processing advantages to avoiding solid-phase polycondensation. Injection molding to make preforms is an example of melt processing. For instance, solid-stated polyester particles develop large shell to core It.V. gradients, which result in larger It.V. losses during melt processing. In addition, solid-stated polyesters have higher melting points due to the annealing that occurs during solid-stating. On the other hand, there is also some quality concerns associated with manufacturing polyesters entirely in the melt phase. Making a high intrinsic viscosity polyester polymer exclusively in the melt phase increases the time exposed to the elevated temperatures necessary to keep the polyester molten, relative to a conventional process with both melt-phase and solid-phase steps. Increasing the time at melt temperatures increases the formation of AA precursors in the molten polyester, which is cooled and formed into polyester particles. During subsequent melt processing of the polyester particles to form articles, the AA precursors, which are present at higher levels, are converted to AA; therefore, high IV polyester particles made entirely in the melt phase have higher AA generation rates upon melting, relative to high IV polyester particles made by a conventional process. It also follows that high IV polyester particles made entirely in the melt phase have higher preform AA levels than high IV polyester particles made by a conventional process using solid state polymerization techniques.
During subsequent melt processing of the polyester particles to form articles such as preforms, the polycondensation catalyst can catalyze the reactions of the AA precursors to form AA. The polycondensation catalyst can also catalyze the reactions to form AA precursors.
There is a need for a polyester polymer built to a high IV entirely in the melt phase and thereby avoids the costly step of solid state polymerization with its melt processing disadvantages, yet generates low amounts of AA upon remelting in the absence of added AA scavengers since scavengers either add cost, black specks or darken/yellow the polymer or all of the above. The need is especially acute in the water bottle market which demands very low levels of AA in preforms and bottles. It would be even more desirable if the same polyester composition can meet the IV requirements of the CSD bottle market.